One example of a wiring board on which electronic components such as semiconductor devices are mounted is a build-up wiring board in which wiring layers and insulating layers are laminated on upper and lower surfaces of a core substrate according to a build-up method in order to increase the density of a wiring pattern.
In the build-up wiring board, a through hole which penetrates the core substrate is formed and a conductive layer is formed on an inner wall surface of the through hole. Wiring layers formed on both surfaces of the core substrate are connected electrically via the conductive layer (see Japanese Laid-Open Patent Publication No. 11-274730). In general, the through hole is filled with a resin.
There has also been proposed a wiring board on which electronic components can be mounted in a position immediately above a through hole in order to expand an area for mounting electronic components on the wiring board. Such a wiring board is provided with a metal layer formed immediately above a through hole (the conductive layer and resin in the through hole).